The present invention relates to an improved bearing bush for center pins on a piano action, and more particularly relates to improvement in a bearing bush for holding center pins used for pivotally supporting swingable parts on a piano action such as a wippen assembly.
As later described in more detail, a piano action includes several swingable parts pivotally mounted to a common center rail via their associated flanges. More specifically, each swingable part is mounted to a center pin each end of which is idly received in a bearing hole of an associated flange via a bearing bush.
Since the piano action is quite frequently driven for operation during performance of a piano, the bearing bush must always assure smooth swing movement of the swingable part supported by the associated center pin. From this point of view, a bearing bush for a piano action must suffice various requirements.
First, a bearing bush is subjected to change in the environmental humidity. When vulnerable to humidity, the bearing bush easily changes its size as the humidity flactuates and such change in size often causes tight fastening of the center pin by the bearing bush, which disenables smooth swing movement of the associated swingable part. Thus, the bearing bush is required to well endure change in the environmental humidity with minimum change in size.
Next, a bearing bush is subjected to frequent, quick and severe frictional contact with the outer surface of the center pin it holds at every swing motion of the associated swingable part. The higher the coefficient of friction, the larger the abrasion of the bearing bush and the center pin. Thus, the bearing bush is required to have a function to lower the frictional contact with the associated center pin as much as possible.
A bearing bush is in general secured in a bearing hole of a flange by bonding agent. When subjected to frequently repeated frictional contact with a center pin, separation tends to occur between the wall surface of the bearing hole and the bearing bush. Once separation starts, the bearing bush can no longer provide stable hold on the center pin and, as a consequence, swing motion of the associated swingable part is seriously disturbed. Thus, the bearing bush is required to keep strong engagement with the wall of the bearing hole in the flange.
Conventionally, it has been proposed to use wool cloth for a bearing bush. More specifically, a fulled wool cloth is cut into strips, i.e. bush cloths, and each bush cloth is formed into a tubular configuration which is then force inserted into bearing holes in a flange after outer coating with proper bonding agent. Thereafter sections of the tubular bush cloth exposed outside the bush holes are removed by shearing.
This wool bearing bush exhibits high elasticity enough to absorb impulsive load during transmission. In addition, inasmuch as it exists between the wall of the bearing hole and the center pin in a highly compressed state, no high accuracy in insertion is required.
However, the material in this case is extremely susceptible to change in the environmental humidity due to the high saturated moisture content of wool. For example, the saturated moisture of wool at 80% relative humidity amounts to 19% and fibers composing the bearing bush undergoes significant swelling which causes serious change in size of the bearing bush, thereby causing a marked change in the holding torque for the center pin.
As a substitute, it is thinkable to make a bearing bush of plastics such as polyester or acrylic resin. Such material is highly impervious to change in the environmental humidity and keeps an almost constant size under different humidity conditions. It further allows no partial separation and/or deformation even at the forced insertion in assembly of the piano action. Despite the advantages, such material lacks in low frictional resistance. Use of lubricative resin of low frictional resistance disenables adherence to bushing holes. Thus, mere use of a bush cloth made of low moisture content synthetic fibers cannot fully suffice the abovedescribed requirements for a bearing bush.